The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing subterranean wells.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone. The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks.
Optimal cement-sheath placement often requires that the cement slurry contain a retarder. Cement retarders delay the setting of the cement slurry for a period sufficient to allow slurry mixing and slurry placement in the annular region between the casing and the borehole wall, or between the casing and another casing string.
A wide range of chemical compounds may be employed as cement retarders. The most common classes include lignosulfonates, cellulose derivatives, hydroxycarboxylic acids, saccharide compounds, organophosphonates and certain inorganic compounds such as sodium chloride (in high concentrations) and zinc oxide. A more complete discussion of retarders for well cements may be found in the following publication—Nelson E B, Michaux M and Drochon B: “Cement Additives and Mechanisms of Action,” in Nelson E B and Guillot D. (eds.): Well Cementing (2nd Edition), Schlumberger, Houston (2006) 49-91.
Certain types of retarders have been blended with other compounds to extend their useful temperature range, improve cement-slurry properties, or both. For example, the useful temperature range of certain lignosulfonate retarders may be extended to more than 260° C. by adding sodium tetraborate decahydrate (borax). Sodium gluconate may be blended with a lignosulfonate and tartaric acid to improve the rheological properties of the cement slurry. Thus, a myriad of retarders and retarder blends exist which may be applicable to a wide range of subterranean-well conditions.
Cement-retarder technology for well cements is sophisticated; however, as exploration and production operations continue to move into environmentally sensitive areas, the population of retarders that may be used is increasingly restricted. This is particularly true in the North Sea. The countries that operate in the North Sea (UK, Norway, Denmark and Holland) maintain a list of chemical products that “pose little or no risk to the environment”. These materials should meet the following criteria. (1) All of the organic components present in the material must be biodegradable in seawater. (2) All of the components should have a low toxicity to fish (Scophthalamus Maximum), marine species (Acartia Tonsa) and algae (Skeletonema Costatum). (3) All of the components should not bioaccumulate. (4) The additive should not contain any prohibited chemicals.
It thus becomes more and more challenging to develop efficient cement retarders (and other types of additives) that can meet these criteria. This is especially true when the cement slurries must be placed in high-pressure/high-temperature (HPHT) wells.
Despite the valuable contributions of the prior art, it would be advantageous to have efficient retarders which perform suitably in HPHT environments. In addition, for logistical reasons in offshore locations, it would be advantageous if the retarders were available in liquid form.